1. Field of the Invention
The present invention generally relates to medical devices and, more particularly, to atherectomy catheter devices.
2. Description of the Related Art
A variety of techniques and instruments have been developed to remove obstructive material in arteries or other body passageways or to repair the arteries or body passageways. A frequent objective of such techniques and instruments is the removal of atherosclerotic plaques in a patient's arteries. The buildup of fatty deposits (atheromas) in the intimal layer (under the endothelium of a patient's blood vessels) characterizes atherosclerosis. Over time, what is initially deposited as relatively soft, cholesterol-rich atheromatous material often hardens into a calcified atherosclerotic plaque. The atheromas may be referred to as stenotic lesions or stenoses while the blocking material may be referred to as stenotic material. If left untreated, such stenoses can so sufficiently reduce perfusion that angina, hypertension, myocardial infarction, strokes and the like may result.
Several kinds of atherectomy devices have been developed for attempting to remove some or all of such stenotic material. In one type of device, such as that shown in U.S. Pat. No. 5,092,873 (Simpson), a cylindrical housing, carried at the distal end of a catheter, has a portion of its side-wall cut out to form a window into which the atherosclerotic plaque can protrude when the device is positioned next to the plaque. An atherectomy blade, disposed within the housing, is then advanced the length of the housing to lance the portion of the atherosclerotic plaque that extends into the housing cavity. While such devices provide for directional control in selection of tissue to be excised, the length of the portion excised at each pass of the atherectomy blade is necessarily limited to the length of the cavity in the device. The length and relative rigidity of the housing limits the maneuverability and therefore also limits the utility of the device in narrow and tortuous arteries such as coronary arteries. Such devices are also generally limited to lateral cutting relative to the longitudinal axis of the device.
Another approach, which solves some of the problems relating to removal of atherosclerotic plaque in narrow and tortuous passageways, involves the use of an abrading device carried at the distal end of a flexible drive shaft. Examples of such devices are illustrated in U.S. Pat. No. 4,990,134 (Auth) and U.S. Pat. No. 5,314,438 (Shturman). In the Auth device, abrasive material such as diamond grit (diamond particles or dust) is deposited on a rotating burr carried at the distal end of a flexible drive shaft. In the Shturman device, a thin layer of abrasive particles is bonded directly to the wire turns of an enlarged diameter segment of the drive shaft. The abrading device in such systems is rotated at speeds up to 200,000 rpm or more, which, depending on the diameter of the abrading device utilized, can provide surface speeds of the abrasive particles in the range of 40 ft/sec. According to Auth, at surface speeds below 40 ft/sec his abrasive burr will remove hardened atherosclerotic materials but will not damage normal elastic soft tissue of the vessel wall. See, e.g., U.S. Pat. No. 4,990,134 at col. 3, lines 20–23.
However, not all atherosclerotic plaques are hardened, calcified atherosclerotic plaques. Moreover, the mechanical properties of soft plaques are very often quite close to the mechanical properties of the soft tissue of the vessel wall. Thus, one cannot always rely entirely on the differential cutting properties of such abrasives to remove atherosclerotic material from an arterial wall, particularly where one is attempting to remove all or almost all of the atherosclerotic material.
Moreover, a majority of atherosclerotic lesions are asymmetrical (i.e., the atherosclerotic plaque is thicker on one side of the artery than on the other). As will be understood, the stenotic material will be entirely removed on the thinner side of an eccentric lesion before it will be removed on the thicker side of the lesion. Accordingly, during removal of the remaining thicker portion of the atherosclerotic plaque, the abrasive burr of the Auth device or the abrasive-coated enlarged diameter segment of the drive shaft of the Shturman device will necessarily engage healthy tissue on the side that has been cleared. Indeed, lateral pressure by such healthy tissue against the abrading device is inherently required to keep the abrading device in contact with the remaining stenotic tissue on the opposite wall of the passageway. For stenotic lesions that are entirely on one side of an artery (a relatively frequent condition), the healthy tissue across from the stenotic lesion will be exposed to and in contact with the abrading device for substantially the entire procedure. Moreover, pressure from that healthy tissue against the abrading device will be, in fact, the only pressure urging the abrading device against the atherosclerotic plaque. Under these conditions, a certain amount of damage to the healthy tissue is almost unavoidable, even though undesirable, and there is a clear risk of perforation or proliferative healing response. In some cases, the “healthy tissue” across from a stenotic lesion may be somewhat hardened by the interaction (i.e., it has diminished elasticity); under such circumstances, the differential cutting phenomenon described by Auth will also be diminished, resulting in a risk that this “healthy” tissue may also be removed, potentially causing perforation.
Thus, notwithstanding the foregoing and other efforts to design a rotational atherectomy device, there remains a need for such a device that can advance through soft atheromas while providing minimal risk of dislodging emboli, and risk of injury to the surrounding vessel wall.